The association of premorbid psychological factors--stress, personality, social environment--and cancer incidence has often been reported as present and occasionally as absent. Reviews of such studies, however, have pointed to possible flaws in their experimental design, with resulting questions about their conclusions. In addition, almost none of the studies has taken into account certain factors having an important impact on outcomes. In these studies, each member of a cohort, presumed initially without cancer, will have produced data on psychological factors on entry. The cohorts will be followed to a point where enough cases of cancer have occurred to detect an excess of deficient risk of cancer in association with specific patterns of psychological factors. When possible, predictions will be made in advance on the basis of prior hypotheses.